At present, nonaqueous electrolyte secondary batteries are attracting attention as motor power supplies for electric tools, electric vehicles (EVs), hybrid electric vehicles (HEVs and PHEVs), and the like in addition to consumer applications including mobile data terminals such as mobile phones, notebook personal computers, and smartphones and are expected to be used for wider applications. Such motor power supplies are required to have increased capacity so as to be used for a long time or enhanced power characteristics in the case of repeating large-current charge and discharge in a relatively short time.
A nonaqueous electrolyte secondary battery in which a lithium-titanium composite oxide is used for a negative electrode active material is stable at high potential and therefore is increasingly expected for novel applications.
Using a lithium-titanium composite oxide for a negative electrode active material reduces the irreversible capacity of a negative electrode. Therefore, in the case of combining the negative electrode with a positive electrode in which a lithium transition metal oxide is used for a positive electrode active material, the irreversible capacity of the positive electrode is generally greater than the irreversible capacity of the negative electrode and discharge cut-off is regulated by the positive electrode in the final stage of discharge. In particular, in the case of using a lithium transition metal oxide having a layered structure for a positive electrode active material, when discharge cut-off is regulated by a positive electrode in the final stage of discharge, the positive electrode active material is likely to be over-discharged; hence, the deterioration of the positive electrode active material is caused in charge-discharge cycles in some cases.
Patent Literatures 1 and 2 disclose that a lithium-titanium composite oxide and another active material such as a carbon material are contained in a negative electrode.